The terrestrial broadcasting transmitter or the base-station transmitter of a mobile telephone incorporates a high-frequency, broadband amplifier circuit that operate in the UHF band or the microwave band. The high-frequency, broadband amplifier circuit has output FET elements in order to amplify the input power to output power. The larger the output power, the lower the output impedance the EFT elements will have.
The FET elements may be connected to an apparatus other than the broadband transmitter to supply its high-frequency output power to the apparatus. If the EFT elements are so used, its impedance, which is low, must be changed to the impedance of the apparatus, which is, for example, 50Ω. Therefore, the high-frequency, broadband amplifier circuit has a push-pull configuration in many cases, so that the EFT elements may operate in the broadband. In any push-pull circuit, two FET elements are connected to two drain power supplies, respectively.
Among the high-frequency signals used in the terrestrial broadcasting are, for example, orthogonal frequency-division multiplexing (OFDM) signal and code-division multiple access (CDMA) signal used in the mobile-telephone base stations. These signals are modulated in a band ranging from several MHz to tens of MHz.
The FET element has its output power and its drain current greatly changed, depending on the high-frequency modulated signal input to it. The inductance component existing between the drain power supply and the FET element deforms the waveform of the drain current. Therefore, various techniques are used to reduce the inductance component.